When refining chromium steel, in particular stainless steel and other chromium steel including at least 9% of chrome, the method of decarburization refining by the AOD method of blowing oxygen gas or a mixed gas of oxygen gas and an inert gas into a melt contained in a refining vessel has been extensively used. In the AOD method, when the decarburization proceeds and the concentration of carbon in the melt drops, the chromium becomes oxidized more easily, so the method has been adopted of raising the ratio of the argon gas or other inert gas in the blown gas along with the drop in the concentration of carbon to suppress the oxidation of chromium. However, in the region of low concentration of carbon, the decarburization rate falls, so a long time is required until reaching the desired concentration of carbon. Further, to raise the ratio of the inert gas in the blown gas, the amount of consumption of the expensive inert gas greatly increases. This is also not advantageous economically.
As a method for promoting the decarburization in the region of low concentration of carbon, utilization of the vacuum refining method may be mentioned. Japanese Unexamined Patent Publication (Kokai) No. 6-287629 discloses the method of supplying oxygen gas or a mixed gas of oxygen gas and inert gas as the blown gas, decarburizing the melt until the concentration of carbon in the melt falls to 0.5 wt %, evacuating in the vessel to not more than 200 Torr (26 kPa), and continuing to decarburize the melt after the concentration of carbon falls below this value. Since performing this treatment under a vacuum from a relative high concentration of carbon and performing the decarburization by a mixed gas with oxygen gas under a vacuum, the oxygen efficiency for decarburization is improved, so the decarburization rate is improved with the same amount of supply of oxygen, the reduction silicon prime units and expensive inert gas prime units can be reduced, and the refining time can be shortened. The pressure inside the vessel in the vacuum treatment is made not more than 200 Torr (26 kPa) because it is considered that the oxygen efficiency for decarburization falls at a pressure higher than that.
Japanese Unexamined Patent Publication (Kokai) No. 9-71809 as well discloses a refining method comprising decarburizing a melt by blowing a gas containing oxygen gas in the atmosphere, then switching from atmospheric treatment to vacuum treatment at the stage when the concentration of carbon drops to 0.7 to 0.05 wt % and blowing a gas containing oxygen gas under a vacuum of 200 (26 kPa) to 15 Torr (2 kPa). The vacuum condition is made not more than 200 Torr (26 kPa) because it is considered the vacuum treatment cannot be effectively performed under a pressure higher than this.
By performing the vacuum treatment in a carbon concentration region of a concentration of carbon of not more than 0.5 wt % or a concentration of carbon of not more than 0.7 wt % and blowing gas containing oxygen gas in the vacuum treatment, it is possible to realize an improvement of the decarburization rate and a reduction of use of the expensive insert gas, but if it were possible to achieve a much shorter refining time or reduced amount of use of inert gas, this would contribute greatly to the reduction of the production costs and improvement of the productivity.
On the other hand, it is extremely difficult to refine ultra-low carbon chromium steel with a concentration of carbon of not more than 0.01% by the AOD method. As the method for promoting decarburization in such a region of low concentration of carbon, utilization of the vacuum refining method may be mentioned. As the utilization of the vacuum refining method, the VOD method of vacuum refining by decarburization in a converter until a suitable concentration of carbon, then shifting the melt to a vacuum refining vessel and the method of using a vacuum AOD furnace for vacuum refining while placing an exhaust hood over the AOD furnace are general.
As an example of the VOD method, Japanese Unexamined Patent Publication (Kokai) No. 51-142410 discloses the method of oxygen refining in a converter, then decarburizing the melt in a vacuum decarburization ladle to make the concentration of carbon after vacuum treatment 0.008%.
As a method using a vacuum AOD furnace, Japanese Examined Patent Publication (Kokoku) No. 60-10087 discloses the method of refining chromium steel by first refining by oxygen gas at the initial ordinary temperature until the carbon falls to about 0.2 to 0.4 wt %, then stopping the supply of oxygen gas while continuing to agitate the melt by the inert gas in the same vessel, continuously lowering the pressure inside the vessel to about 10 Torr (1.3 kPa), and lowering the concentration of carbon after vacuum treatment to 0.13 wt %.
With the above method, the carburization under vacuum uses only inert gas, so the oxidation of chromium is suppressed, but the oxygen source of the decarburization becomes the oxygen in the melt or the oxygen in the slag and the rate of supply of oxygen becomes slow, so a drop in the decarburization rate is invited. Therefore this cannot be said to be an efficient decarburization refining method. As opposed to this, Japanese Unexamined Patent Publication (Kokai) No. 6-287629 discloses a decarburization refining method for chromium-contained molten steel comprising supply a mixed gas of oxygen gas and inert gas as the blown gas, performing decarburization refining under atmospheric pressure until the concentration of carbon in the melt falls to 0.5 wt %, then, after the concentration of carbon falls below this value, evacuating the inside of the vessel to not more than 200 Torr (26 kPa) and continuing to decarburize the melt. In this method, gas including oxygen gas is supplied even in the vacuum refining. Due to this, the oxygen efficiency for decarburization is improved, so an improvement in the decarburization rate is achieved and the refining time can be shortened, so it is possible to achieve a large reduction in the refining costs and improvement in the productivity and refining down to the ultra-low carbon region of a concentration of carbon of not more than 0.01 wt % becomes easy. In this invention, the total amount of the blown gas during the vacuum annealing is made 0.3 Nm3/min·T.
In decarburization refining of ultra-low carbon chromium-contained molten steel, by applying vacuum refining to the decarburization in the low carbon concentration region and using a gas containing oxygen gas as the bottom blown gas used at the time of vacuum refining, refining of the ultra-low carbon area of a concentration of carbon of not more than 0.01 wt % becomes possible, but the decarburization rate gradually falls along with the fall in the concentration of carbon, so to decarburize the melt until this ultra-low carbon region, an extremely long refining time is required compared with decarburization refining down to the ordinary low carbon region. Therefore, compared with usual refining of low carbon chromium steel, a drop in productivity of the decarburization refining is invited and an increase in the refining costs is caused.
Further, regarding the refining apparatus for a chromium-contained molten steel, vacuum refining furnaces comes in various types such as VOD, AOD, RH, and REDA, but vacuum exhaust equipment is required for evacuating the inside of the furnace. The vacuum exhaust equipment for industrially evacuating the inside of a vacuum refining furnace generally achieves a predetermined degree of vacuum inside the furnace by combining a large number of ejectors. The degree of vacuum is controlled in accordance with the progress in refining in the vacuum refining furnace, but normally one or more ejectors with capacities commensurate with the targeted degree of vacuum are operated among a large number of ejectors to secure the predetermined degree of vacuum.
On the other hand, one type of vacuum exhaust unit used industrially is a water-sealed vacuum pump. When using this alone, due to the problem of cavitation, the attainable degree of vacuum is about 61 Torr (8 kPa). To obtain a higher degree of vacuum, it is necessary to jointly use the above-mentioned ejectors.
When controlling the degree of vacuum using only ejectors, nitrogen, air, etc. is blown in before the ejectors and the blow rate is controlled so as to control the degree of vacuum in the furnace or the ducts.
When refining a melt using gaseous oxygen under vacuum, the CO gas produced by the decarburization reaction causes the metal and slag to splash from the surface of the melt toward the top of the vacuum refining furnace. The amount of this generated increases sharply when the degree of vacuum rises (when a high vacuum is reached) and deposits on the alloy addition port, furnace cover, ducts, etc. at the top of the refining vessel to block the same or cause trouble in various equipment and operations and obstruct productivity. If raising the degree of vacuum and increasing the oxygen blow rate, a rapid decarburization reaction will proceed and the phenomenon will arise of the CO gas generated causing a large amount of metal to be blown upward all at once from near the surface of the melt, that is, boiling will be caused. This will also become major trouble in the equipment and worsen the productivity.
In this way, vacuum oxygen decarburization of a carbon melt is an operation which requires extreme care. The point is to control the degree of vacuum and the oxygen blow rate in accordance with the concentration of carbon in the melt. Among these, the oxygen blow rate can be controlled to a certain extent by the flow adjustment valve of the oxygen gas, but no sufficient control method has been established for the degree of vacuum.
In the above prior art, when using ejectors, the method of successively starting and stopping a large number of ejectors does not allow extremely fine control of the degree of vacuum since the ranges of capacity of the ejectors themselves are broad. Further, as seen in Japanese Unexamined Patent Publication (Kokai) No. 10-1716, the method of allowing gas to leak in from the outside while operating the exhaust unit (for example, using nitrogen) enables control of the degree of vacuum to a certain extent, but has the defect that the gas costs rise. As a means for slashing the gas costs, there is the method of using air as an alternative to nitrogen. However, while control of the degree of vacuum itself is possible, the exhaust gas sucked in contains a high concentration of CO gas, so when mixing in air containing a combustion-assisting gas constituted by oxygen, there is the danger of combustion and explosion. Employment for actual machinery is extremely dangerous. Further, if allowing gas to leak in from the outside, the load on the exhaust unit increases. For example, the power used by the vacuum pump increases. Therefore, this is not preferable from the viewpoint of energy conservation. Further, the method of controlling the amount of supply of steam to an ejector used in this patent relies on the fact that the optimum steam flow rate of an ejector is distinctive, so changing this remarkably reduces the exhaust performance of the ejector itself. Further, at the same time, a slight fluctuation in the amount of steam is overly sensitively reflected in the ejector performance, so extremely fine control of the pressure inside the refining vessel becomes difficult.
On the other hand, the method of using a water-sealed type vacuum pump is currently employed for control of the degree of vacuum by pump units, but this is not used together with ejectors, the capacity is insufficient for realizing a high vacuum by this alone, and extremely fine control of the degree of vacuum is impossible.
Further, in a vacuum refining vessel, in most cases, for efficient refining or for final adjustment of the ingredients of the melt, alloy or secondary materials are added to the melt in the middle of refining or at the end stage of refining. Normally, these are charged into the vessel and added to the melt by allowing them to naturally drop from an alloy hopper provided at the top of the refining vessel through a chute.
However, due to the argon blown into the refining vessel for agitating the melt or the oxygen blown for promoting decarburization, splash of the metal and slag, generation of dust, etc. occur inside the refining vessel. Therefore, the metal deposits at the alloy and secondary material addition port linked with the inside of the vessel and accordingly the addition port becomes blocked or other trouble easily occurs. Therefore, to suppress the occurrence of such trouble, the means has been adopted of providing the alloy and secondary material addition port with side walls resistant to the effects of the metal and slag or, in the case of a refining vessel with a high tank height, providing a top cover. Further, the means has also been adopted of using the alloy and secondary material addition port jointly as the insertion port of the top blowing lance. If considering continuous long term operation of a vacuum refining vessel, however, neither means is sufficient in practice.
Further, in treatment of the exhaust gas of a metallurgical furnace, including atmospheric and vacuum refining vessels, it is necessary to cool the high temperature exhaust gas produced. Therefore, sometimes a water-cooled type gas scrubber is provided in the middle of the ducts or the ducts are water cooled in the middle. In this case, heat is exchanged between the high temperature exhaust gas and the large amount of cooling water. Due to abrasion and reduced thickness of the piping and ducts, cracking due to thermal stress, etc., sometimes the cooling water leaks from the piping and ducts to the inside of the exhaust gas passage. Exhaust gas treatment equipment is generally closed, however, so it is impossible to obtain a grasp of the state of water leakage inside. Therefore, sometimes operation is continued while not being able to confirm internal water leakage and the water leakage becomes serious and leads to a remarkable drop in the degree of vacuum or the inability to remove dust from the system due to the water leakage or other trouble in equipment or operation.
Therefore, operation has been stopped on a scheduled basis at a certain frequency and the inside of the ducts checked and the gas cooler checked. Further, the practice has been to install an electrostatic capacity type detection rod at the dust collector at the bottom of the gas cooler and utilize the fact that dust changes in electrostatic capacity when wet by water leakage so as to detect water leakage.
If stopping operation and conducting checks on a scheduled basis, however, the operating efficiency of the facilities will be reduced and the productivity blocked. On the other hand, with the above-mentioned electrostatic capacity type detection rod, it is difficult to adjust the electrostatic capacity of the detection rod according to the state of wetness of the dust. For example, with a small amount of water leakage, if the temperature is high or under a vacuum, the water will easily turn into steam, so detection of water leakage will not be possible. The detection rod is predicated on detection of a large amount of water leakage. Therefore, it is extremely difficult to detect water leakage in advance while still slight.
Further, vacuum exhaust equipment for industrially evacuating a vacuum refining vessel generally achieves a predetermined degree of vacuum in the furnace by combining a large number of ejectors or using a vacuum pump. Vacuum ejectors utilize the so-called “mist-blowing principle” and suck in and exhaust the exhaust gas in the vacuum refining vessel and the ducts and other parts of the vacuum path by the ejected media. For the ejected medium, usually steam is used industrially. Steam is condensed by the cooling water at a condenser after the ejectors to become water again and therefore only the exhaust gas is exhausted to the next stage. The cooling water of the condenser and the condensed water of the steam are temporarily collected and stored at a water storage tank near the ground and are pumped to the cooling tower by a pump. On the other hand, as the vacuum pump, industrially a water-sealed pump is used and a large amount of water is used. The water used by the vacuum pump is collected and stored in a water storage tank in the same way as the condenser water.
Exhaust gas contains a large amount of CO gas. The condenser water is accompanied by large numbers of bubbles of exhaust gas containing CO which flow into the water storage tank along with it. Therefore, the inside of the water storage tank becomes an atmospheric gas containing CO gas in composition. In the sense of preventing the gas inside the tank from leaking outside the tank, closeability and sealability are very important as functions required for a water storage tank.
Water storage tanks come in generally two types: steel seal pots and concrete (the top cover part made of steel) hot wells. Steel seal pots have a good closeability, but suffer from the problems of corrosion and swelling capital costs. On the other hand, concrete hot wells are free from corrosion and relatively inexpensive in terms of capital costs as well, but suffer from problems in the sealability with the top steel covers. In the following description, the invention will be explained taking as an example mainly the latter concrete hot wells, but the invention may similarly be applied to steel seal pots.
There are two issues with hot wells. The first is that there is leakage of CO-containing gas from a hot well. The second is the suppression of damage to the equipment when the cooling water inside a hot well overflows.
As means for dealing with this, the method of forcibly evacuating the inside of the hot well by a suction fan is widely employed. Due to this, the inside of the hot well becomes a constantly negative pressure and the danger of leakage of the inside gas is remarkably reduced. However, the inside of a hot well being made negative pressure due to suction of gas means suction of air from the seal parts. The clearance of the seal parts therefore gradually expands. If the suction fan were to stop in this state for some reason or another, a large amount of CO-containing gas would leak from the expanded clearance of the seal parts.
Further, even if the power of the system of the return pump of the hot well is cut off for some reason and the return pump stops, the supply pump of the large-sized cooling tower will continue to operate. This being so, the cooling water in the hot well will continue to increase and will overflow. As a measure against this, it may be considered to attach a switch valve from another power source system to the supply pipe to the condenser and water-sealed pump, but tremendous expense would become required for the long distance pipeline and the large switching valve.